1. Technical Field
The present invention relates generally to piezoelectric damping systems. More particularly, the invention relates to a passive piezoelectric damping system having a shunt inductance tuned to a non-resonant mode of a structure.
2. Background Art
Interior noise control is of vital importance in aircraft, rotorcraft, launch vehicles, automobiles and many other engineering applications. The noise levels in commercial and military aircraft and helicopters can be very high and can result in passenger discomfort, interference with communications, and crew fatigue. A variety of noise sources and transmission paths contribute to cabin noise. Sources such as propellers, rotors, inlet and exhaust systems, machinery and engines generate noise that impinges directly on the exterior of the fuselage and transmits into the cabin. Such noise is referred to as xe2x80x9cairborne noisexe2x80x9d. Control of the interior noise environment requires significant effort, and the noise control measures usually result in substantial added weight and reduced performance.
Passive noise control treatments, such as lead vinyl sheets, have been used to attenuate low frequency airborne noise transmission. Higher frequency airborne noise is usually controlled by acoustically absorptive treatments, e.g., fiberglass blankets. These methods, however, add considerable weight to the aircraft, thereby affecting aircraft performance and operational cost. In order to reduce weight and manufacturing costs, new methods must be developed which are simpler and require minimum maintenance.
Some specific examples where passive piezoelectric elements can be readily used are aircraft and rotorcraft. The cabin noise environment in a rotorcraft is, in general, unbearable and is dominated by intense tones which come from rotors and other sources. Low frequency tonal noise is becoming a problem in new generation aircraft in which engines are mounted directly to the wing. The engines on some aircraft are mounted on a pylon in close proximity to the airframe thereby transmitting even more intense engine tones inside the cabin. Similar severe vibration and noise problems also exist in the space station and launch vehicles. It is therefore desirable to provide new noise control treatments to reduce cabin noise.
Airborne noise transmission in the low to mid-frequency range, in particular, is a very difficult problem to solve as it responds only to large increases in the weight of the structure. FIG. 2 demonstrates a conventional mechanical damping system to better illustrate the difficulties associated with conventional approaches. It can be seen that the mechanical damping system (with a single degree of freedomxe2x80x94SDOF) has a mass 14xe2x80x2 responding to an input force Fin, a spring 32xe2x80x2, and a viscous damper 34xe2x80x2. An electrical circuit representation of the mechanical damping system in FIG. 2 is shown in FIG. 3. With continuing reference to FIGS. 2 and 3, it can be easily shown that the response and input mobility (Y=v/F, where v is velocity response and F is input force) of the SDOF system at the resonance frequency is inversely proportional to the damping loss factor. At resonance, the reactance of the mass 14xe2x80x2 is canceled by the inductance of the spring 32xe2x80x2 in the mechanical system. The balancing of the mass reactance with the inductance occurs within a very narrow frequency range (called the half-power bandwidth) in the vicinity of the resonance frequency. Consequently, the mechanical mobility of a SDOF mechanical system at resonance can be represented by a pure dashpot 34xe2x80x2 or as an outside resistor 34 in the electrical circuit representation. The vibration energy associated with the resonance mode in the mechanical system is, therefore, able to flow and get dissipated in the outside resistor 34. The path for the mechanical energy to flow from the structure to an electrical circuit is provided by a piezoelectric element, which is bonded to the structure and converts mechanical energy into electrical energy.
The input mechanical mobility of a structure for non-resonant transmission, however, is very different from that described for the resonant transmission. A piezoelectric shunt circuit tuned at a resonance frequency will not provide a matching electro-mechanical circuit for the NR modal energy to flow through. It can be shown that the mechanical mobility of an NR mode, which is spatially excited at a frequency much higher than its natural frequency, is mostly capacitive reactance as opposed to simply resistive reactance for a resonant mode. In fact, the inductance (or stiffness) of the NR mode is almost non-existent at the driving frequency, and does not balance the capacitive reactance of the NR mode. The mechanical energy of an acoustically-fast NR mode is therefore stored in the capacitive reactance and is released only as radiated sound in the ambient medium. Furthermore, the reactance of the piezoelectric element adds to the total capacitive reactance and must be accounted for in any efficient damping system. It is therefore desirable to provide a new approach to reducing airborne noise transmission (resulting in the excitation of NR modes) using passive piezoelectric elements which do not add substantial weight to the structure.
The above and other objectives are provided by a system and method in accordance with the present invention for dissipating mechanical energy propagating through a structure. The passive piezoelectric damping system of the present invention includes a piezoelectric element coupled to the structure, where the piezoelectric element converts the mechanical energy into electrical energy. The electrical energy has a reactive component. The damping system further includes a shunt circuit connected to the piezoelectric element for balancing the reactive component of the electrical energy with a shunt inductance. The shunt inductance is tuned to a non-resonant mode of the structure. Tuning the shunt inductance to a non-resonant mode of the structure allows dissipation of mechanical energy resulting from airborne noise without significantly increasing the mass of the structure.
Further in accordance with the present invention, a method for dissipating mechanical energy propagating through a structure is provided. The method includes the step of converting the mechanical energy into electrical energy with a piezoelectric element, where the electrical energy has a reactive component. A shunt inductance is tuned to a non-resonant mode of the structure and the reactive component of the electrical energy is balanced with the shunt inductance.
The present invention also provides a method for tuning a shunt inductance to a non-resonant mode of a structure. The method includes the step of determining a reactive component of electrical energy from a piezoelectric element coupled to the structure. The shunt inductance is then estimated based on the reactive component. The method further includes the step of selecting an inductor having the shunt inductance. Further in accordance with the present invention, the reactive component of the electrical energy can be either estimated or measured.